Erect Macroalgae Research Articles

Microbiomes and metabolomes of dominant coral reef primary producers illustrate a potential role for immunolipids in marine symbioses

The dominant benthic primary producers in coral reef ecosystems are complex holobionts with diverse microbiomes and metabolomes. In this study, we characterize the tissue metabolomes and microbiomes of corals, macroalgae, and crustose coralline algae via an intensive, replicated synoptic survey of a single coral reef system (Waimea Bay, Oʻahu, Hawaii) and use these results to define associations between microbial taxa and metabolites specific to different hosts. Our results quantify and constrain the degree of host specificity of tissue metabolomes and microbiomes at both phylum and genus level. Both microbiome and metabolomes were distinct between calcifiers (corals and CCA) and erect macroalgae. Moreover, our multi-omics investigations highlight common lipid-based immune response pathways across host organisms. In addition, we observed strong covariation among several specific microbial taxa and metabolite classes, suggesting new metabolic roles of symbiosis to further explore.

Compounded effects of sea urchin grazing and physical disturbance on macroalgal canopies in the lagoon of Moorea, French Polynesia

Release from herbivory is a factor underpinning coral replacement by macroalgae. Once macroalgae have achieved dominance, shifts back to the coral-dominated state can be hindered by stabilizing feedbacks. Thus, restoring herbivore assemblages alone can be insufficient to trigger coral recovery. However, herbivores could control macroalgal recovery in the aftermath of physical disturbances removing macroalgae. Diadematid urchins at Moorea (French Polynesia) have collapsed in the last decade. By means of a manipulative field experiment, we tested the interactive effects of physical disturbance and increased diadematid densities on macroalgae inside the lagoon. Massive Porites colonies, referred to as ‘bommies’, were assigned to 3 different macroalgal removal treatments (removal of stipes and fronds of the canopy-forming macroalgae Turbinaria ornata and Sargassum pacificum, total removal of erect macroalgae or untouched) and exposed to 3 different urchin densities (absent, low [~0.5 ind. m-2], and intermediate [~1 ind. m-2]). After 1 yr, sea urchins had no effect on the covers of S. pacificum and T. ornata when macroalgal canopies were left untouched. Urchins could control the recovery of S. pacificum on total macroalgal removal bommies, but not that of T. ornata. However, urchins, even when at intermediate densities, did not generate major changes in the structure of benthic assemblages on experimental bommies. Our study indicates that a moderate increase in diadematid densities is unlikely to reduce the extent of macroalgal stands in Moorea back reefs unless associated with the recovery of other herbivore guilds able to remove adult macroalgae (i.e. browsers).

Drivers of damselfishes distribution patterns in the southwestern Atlantic: tropical and subtropical reefs compared

Abstract Damselfishes are known keystone species of reef environments, however large-scale distribution patterns are poorly studied in the southwestern Atlantic. We evaluated main drivers of distribution of three conspicuous damselfishes, along tropical and subtropical coastal systems, in Brazil. Abundances were assessed against wave exposure, depth (within 1–7 m in tropical and 1–11 m in subtropical reefs) and benthic cover. Despite differences between systems, exposure and depth consistently explained damselfishes distribution. Stegastes fuscus, the larger damselfish species of the genus in the southwestern Atlantic, was dominant in both systems, inhabiting preferably shallow and sheltered reefs. Conversely, Stegastes variabilis occupied shallow habitats with higher exposure. Stegastes pictus was absent from tropical reefs sampled, inhabiting depths >7 m, in subtropical reefs. Species were weakly associated with benthic features, which poorly predicted changes in abundances. Regardless, S. fuscus showed association with articulated calcareous algae, and S. variabilis juveniles associated with erect macroalgae. Despite occurring in very distinctive reef systems, Brazilian damsels habitat requirements are consistent in both tropical and subtropical reefs. While highly persistent species, long term monitoring will inform us how they respond to pervasive global changes and human impacts along Brazilian reefs.

Taxonomic composition of mobile epifaunal invertebrate assemblages on diverse benthic microhabitats from temperate to tropical reefs

Anthropogenic drivers are flattening reef structure from 3-dimensional habitats composed of macroalgae and live branching corals towards low-profile turfing algae. Our current understanding of the consequences of widespread reef degradation currently fails to consider the responses of small mobile invertebrates (‘epifauna’) to patterns of change amongst reef structural elements (‘microhabitats’). Here, the taxonomic composition of 152 epifaunal assemblages was compared among 21 structurally diverse benthic microhabitats across an Australian temperate to tropical climatic gradient, spanning 28.6 degrees in latitude from Tasmania to the northern Great Barrier Reef. Epifauna varied consistently with different microhabitat types, and to a much lesser extent with latitude. Macroalgae, live branching coral and turfing algae represented 3 extremes for epifaunal community structure, with most microhabitats possessing epifaunal assemblages intermediate between these endpoints. Amongst structural characteristics, epifauna related primarily to the degree of branching and hardness of microhabitats. Mobile invertebrate communities are likely to transform in predictable ways with the collapse of large erect macroalgae and live coral towards low-lying turf-associated communities.

Biophysical Habitat Features Explain Colonization and Size Distribution of Introduced Trochus (Gastropoda)

The trochid gastropod, Rochia nilotica is harvested in small-scale fisheries throughout Southeast Asia and the Pacific Islands, many of which were created from inter-country translocations. This species is found on structurally complex shallow reefs, but specific habitat requirements have not yet been quantified in order the maximize the success of future translocations and help future-proof the fishery in changing ocean conditions. At 28 sites around Samoa, where the species was introduced in the early 2000s, we measured a suite of habitat variables along transects in which R. nilotica were counted and the shell sizes measured in a parallel study. Boosted regression tree analyses revealed that R. nilotica were most abundant at reef locations that were shallow, with fairly consistent depth, high cover of branching coral, low cover of erect macroalgae, low wave exposure and high surface complexity. Smaller individuals were associated with wide reef flats and high cover of branching coral, whereas larger animals occurred in deeper water with high surface complexity. Multivariate analyses showed this species to be a habitat generalist, sharing much of its niche with an endemic herbivorous gastropod, Tectus pyramis. Future stocking programs should focus on sites with habitats optimal for both adults and juveniles. R. nilotica populations are likely to be especially affected by broadscale stressors that result in declines in live coral cover and substratum complexity and increasing coverage of macroalgae on coral reefs.

Selective top-down control of epiphytic biomass by amphipods from Posidonia oceanica meadows: implications for ecosystem functioning

Mediterranean Posidonia oceanica meadows shelter an important biomass and biodiversity of amphipod crustaceans that graze on epiphytes. However, their actual significance for ecosystem functional processes is hard to estimate, due to the lack of adequate data. Here, a field microcosm-based inclusion experiment was used to test if three of the dominant taxa of the amphipod community (Apherusa chiereghinii, Dexamine spiniventris and Gammarus spp.) could exert top-down control on seagrass leaf epiphytes. Influence of amphipod activity on nutrient availability for the host species was also investigated. All grazer taxa significantly reduced biomasses of erect macroalgae and erect sessile animals present on leaves. None of them consumed encrusting epiflora or epifauna. This selective top-down control could have important implications for the structure of the epiphytic community on leaves of P. oceanica, which is one of the most diverse and abundant of all seagrass species. Grazing activity of all taxa caused higher N content of seagrass leaves, likely through amphipod excretion and/or sloppy feeding. Since P. oceanica meadows often grow in oligotrophic zones where plant growth can be nutrient-limited, this N enrichment could enhance seagrass production. Overall, the ecological interaction between P. oceanica and amphipods could be seen as a facultative mutualistic relationship. Our results suggest that amphipod mesograzers are key-elements in some of the functional processes regulating these complex and yet endangered ecosystems, which are essential components of Mediterranean coastal zones.

Macroalgal community response to herbivores and sediment deposition: an indicator of coral reef degradation

The effects of herbivores and sediment deposition on the structure and composition of the macroalgal communities on coral reefs were tested together experimentally and, in the field, in one of the most disturbed reefs of the Caribbean, the leeward reefs of the Bocas del Toro region, Panama. In this area, the potential natural herbivores have been replaced by small-bodied herbivores, mainly the urchin Echinometra viridis, and sediment deposition has increased as a result of land use in the zones close to the reefs. Experimental studies consisted of herbivorous exclusion and artificial increase in sediment deposition; while, in different sites of the studied area, relationships between macroalgal communities, herbivore abundance and sediment deposition were examined in situ. Both approaches suggest that changes on macroalgal species and the structure of the communities are mediated by herbivores and sediment deposition. The experimental approach showed an interaction between the studied factors. Sediment deposition, at the levels used in the experiment, did not limit the grazing of E. viridis. This urchin controlled the development of communities, mainly the erect macroalgae, independently of the level of sediment deposition tested. However, with the treatment of herbivores present and increase of sediment deposition, Ceramiales turf replaced cyanophytes. This contrasted with our findings in the field, where extreme values of sediment deposition coincided with minimal urchin populations. These last conditions favoured communities dominated by erect macroalgae, but their composition and abundance differ from those of communities that develop under low herbivory and low sedimentation because the deposited sediments also influence the community structure.

Spatial partitioning of Zoantharian species (Suborder Brachycnemina) at intertidal rocky platforms of the Canary Islands

Spatial partitioning of Zoantharian species (Suborder Brachycnemina) at intertidal rocky platforms of the Canary Islands

Erect macroalgae influence epilithic bacterial assemblages and reduce coral recruitment

Erect macroalgae influence epilithic bacterial assemblages and reduce coral recruitment

Influencia humana en las fluctuaciones poblacionales de erizos de mar

Echinoids play an important role in marine ecosystems structuring. Often, their population density experience markedly fluctuations that promote a state shift in the ecosystems they inhabit. Population increments of some sea urchins may cause catastrophic changes in temperate areas of the planet by decimating the erect macroalgae cover. These population increments results in unproductive and very stable assemblages, known as “blanquizales” (ericeras/moradales), or sea urchin barren ground. Macroalgae are the main ecosystem engineers in temperate areas and generate a suitable nursery and feeding habitat for fishes. These algae stands are also important zones for biofiltration of coastal waters and CO2 uptake. The main consequence of vegetated biomass lost is a trophic disequilibrium that generates important economic losses for artisanal fisheries and tourism. In tropical areas, sea urchin’s outbreaks can cause bioerosion in coral reefs. However, the most important event to highlight was the mass mortality occurred in the Caribbean during the 80’s. This event favored the development of algae communities that suffocated the coral reef ecosystem. Therefore, both in temperate and tropical areas of the planet, these boom-bust echinoids generate undesired ecosystems states. Very recently, various global scale collaborative papers have highlighted a clear anthropogenic cue. Human activity though overfishing or favoring global warming, weakens marine ecosystem resilience and promote these catastrophic ecosystem shifts. To mitigate the effects of these population changes different management strategies have been used. For instance, in temperate areas, sea urchin reduction actions (manually or by using quick lime), sea urchin harvesting or the implementation of marine reserves have been used, with contrasting results. In Caribbean coral reefs affected by urchin mass mortality, some sea urchin juvenile’s reintroduction plans have been used but with very low effectiveness. The more feasible ecosystem scale strategy due to its preventive nature, seems to be the implementation of protected areas. However, the impact of global warming will exceed our capacity to manage marine ecosystems locally and will required more efficient global actions to prevent undesired sea urchin fluctuations. Rev. Biol. Trop. 65(Suppl. 1): S23-S34. Epub 2017 November 01.

Combined impacts of natural and human disturbances on rocky shore communities

Most ecosystems are subject to both natural and human disturbances that can combine to influence populations and assemblages in complex ways. Assessing the relative influences and combined impacts of natural and human disturbance is crucial for managing human uses of ecosystems against the backdrop of their natural variability. We evaluated the separate and combined influences of disturbance from storm waves and disturbance associated with human trampling of rocky shores by conducting an experiment mimicking controlled levels of trampling at sites with different wave exposures, and before and after a major storm event in central California, USA. Results show that trampling and storm waves affected the same taxa and have comparable and additive effects on rocky shore assemblages. Both disturbance types caused significant reduction in percent cover of mussels and erect macroalgae, and resulted in significant re-organization of assemblages associated with these habitat-forming taxa. A single extreme storm event caused similar percent cover losses of mussels and erect macroalgae as did 6–12 months of trampling. Contrary to a predicted synergistic effect of trampling and storm damage, we found that impacts from each disturbance combined additively. Mussel beds in wave-exposed sites are more vulnerable to trampling impacts than algal beds at protected sites. Mussels and erect macroalgae recovered within five years after trampling stopped. These results suggest that impacts from local human use can be reversed in relatively short time frames, and that cumulative impacts can be reduced by setting recreational carrying capacities more conservatively when ecosystems are already exposed to frequent and/or intense natural disturbances.

The real bounty: marine biodiversity in the Pitcairn Islands.

In 2012 we conducted an integrated ecological assessment of the marine environment of the Pitcairn Islands, which are four of the most remote islands in the world. The islands and atolls (Ducie, Henderson, Oeno, and Pitcairn) are situated in the central South Pacific, halfway between New Zealand and South America. We surveyed algae, corals, mobile invertebrates, and fishes at 97 sites between 5 and 30 m depth, and found 51 new records for algae, 23 for corals, and 15 for fishes. The structure of the ecological communities was correlated with age, isolation, and geomorphology of the four islands. Coral and algal assemblages were significantly different among islands with Ducie having the highest coral cover (56%) and Pitcairn dominated by erect macroalgae (42%). Fish biomass was dominated by top predators at Ducie (62% of total fish biomass) and at Henderson (35%). Herbivorous fishes dominated at Pitcairn, while Oeno showed a balanced fish trophic structure. We found high levels of regional endemism in the fish assemblages across the islands (45%), with the highest level observed at Ducie (56% by number). We conducted the first surveys of the deep habitats around the Pitcairn Islands using drop-cameras at 21 sites from depths of 78 to 1,585 m. We observed 57 fish species from the drop-cams, including rare species such as the false catshark (Pseudotriakis microdon) and several new undescribed species. In addition, we made observations of typically shallow reef sharks and other reef fishes at depths down to 300 m. Our findings highlight the uniqueness and high biodiversity value of the Pitcairn Islands as one of the least impacted in the Pacific, and suggest the need for immediate protection.

Grazer removal and nutrient enrichment as recovery enhancers for overexploited rocky subtidal habitats.

Increasing anthropogenic pressures are causing long-lasting regime shifts from high-diversity ecosystems to low-diversity degraded ones. Understanding the effects of multiple threats on ecosystems, and identifying processes allowing for the recovery of biodiversity, are the current major challenges in ecology. In several temperate marine areas, large parts of rocky subtidal habitats characterised by high diversity have been completely degraded to barren grounds by overfishing, including illegal date mussel fishing. Bare areas are characterized by the dominance of sea urchins whose grazing perpetuates the impact of overfishing. We investigated experimentally the separate and combined effects of nutrient enrichment and sea urchin exclusion on the recovery of barren grounds. Our results indicate that the two factors have a synergistic effect leading to the re-establishment of erect macroalgal canopies, enhancing the structural complexity of subtidal assemblages. In particular, in the overfished system considered here, the recovery of disturbed assemblages could occur only if sea urchins are removed. However, the recolonization of barren grounds by erect macroalgae is further enhanced under enriched conditions. This study demonstrates that the recovery of dramatically depleted marine habitats is possible, and provides useful indications for specific management actions, which at present are totally lacking, to achieve the restoration of barren grounds caused by human activity.

Sea urchins predation facilitates coral invasion in a marine reserve.

Macroalgae is the dominant trophic group on Mediterranean infralittoral rocky bottoms, whereas zooxanthellate corals are extremely rare. However, in recent years, the invasive coral Oculina patagonica appears to be increasing its abundance through unknown means. Here we examine the pattern of variation of this species at a marine reserve between 2002 and 2010 and contribute to the understanding of the mechanisms that allow its current increase. Because indirect interactions between species can play a relevant role in the establishment of species, a parallel assessment of the sea urchin Paracentrotus lividus, the main herbivorous invertebrate in this habitat and thus a key species, was conducted. O. patagonica has shown a 3-fold increase in abundance over the last 8 years and has become the most abundant invertebrate in the shallow waters of the marine reserve, matching some dominant erect macroalgae in abundance. High recruitment played an important role in this increasing coral abundance. The results from this study provide compelling evidence that the increase in sea urchin abundance may be one of the main drivers of the observed increase in coral abundance. Sea urchins overgraze macroalgae and create barren patches in the space-limited macroalgal community that subsequently facilitate coral recruitment. This study indicates that trophic interactions contributed to the success of an invasive coral in the Mediterranean because sea urchins grazing activity indirectly facilitated expansion of the coral. Current coral abundance at the marine reserve has ended the monopolization of algae in rocky infralittoral assemblages, an event that could greatly modify both the underwater seascape and the sources of primary production in the ecosystem.

The seaweed Caulerpa racemosa on Mediterranean rocky reefs: from passenger to driver of ecological change

Disentangling the ecological effects of biological invasions from those of other human disturbances is crucial to understanding the mechanisms underlying ongoing biotic homogenization. We evaluated whether the exotic seaweed, Caulerpa racemosa, is the primary cause of degradation (i.e., responsible for the loss of canopy-formers and dominance by algal turfs) on Mediterranean rocky reefs, by experimentally removing the invader alone or the entire invaded assemblage. In addition, we assessed the effects of enhanced sedimentation on the survival and recovery of canopy-forming macroalgae at a relatively pristine location and how their loss affects the ability of C. racemosa to conquer space. C. racemosa did not invade dense canopy stands or influence their recovery in cleared plots. Competition with C. racemosa could not explain the rarity of canopy-forming species at degraded sites. Removing the assemblages invaded by C. racemosa and preventing reinvasion did not trigger the transition from algal turfs to canopies, but it enhanced the cover of morphologically complex erect macroalgae under some circumstances. Once established, C. racemosa, enhancing sediment accumulation, favors algal turfs over erect algal forms and enables them to monopolize space. Our results show that introduced species that rely on disturbance to establish can subsequently become the main drivers of ecological change.

Among-habitat variation in herbivory on Sargassum spp. on a mid-shelf reef in the northern Great Barrier Reef

Abstract Herbivory is widely acknowledged as a key process determining the benthic community structure and resilience of coral reefs. Despite numerous studies that have examined herbivory across reef gradients in the Caribbean, few studies have directly quantified this process on Pacific reefs. Bioassays of two species of erect macroalgae (Sargassum swartzii and S. cristaefolium) were used to quantify variation in grazing intensity across seven habitats of varying depth and wave exposure on a mid-shelf reef in the northern Great Barrier Reef. Removal rates of Sargassum varied significantly among habitats, with both species displaying broadly similar patterns. The shallow habitats on the exposed aspect of the reef (i.e. reef crest, flat and back reef) experienced the highest reductions in mass (81.4–91.6% day-1) for both S. swartzii and S. cristaefolium, while the deeper exposed habitats (reef slope and base) displayed the lowest reductions (3.8– 13.4% day-1) over a 24 h period. In contrast, the grazing intensity varied between the two species in the three habitats on the leeward aspect of the reef. Reductions in mass remained relatively high for S. swartzii on the patch reef and sheltered reef base and flat (62.7–76.5% day-1) but were considerably lower for S. cristaefolium (37.9– 63.5% day-1) across the same habitats. Surprisingly, the rates of removal of Sargassum displayed no relationship with the density or biomass of roving herbivorous fishes or those species known to consume erect macroalgae, either collectively or independently. These results suggest that the relationship between browsing rates and herbivorous fish biomass is complex and may be driven by species that are underestimated in visual surveys. Direct quantification of browsing intensity using assays revealed a different pattern to inferences based on herbivore densities and highlights the potential difficulties of evaluating ecosystem processes based on visual census data alone.

Fish versus starfish predation in controlling sea urchin populations in Mediterranean rocky shores

In the Mediterranean, fishing bans generally allow the recovery of populations of sea urchin predators, such as the seabreams Diplodus sargus and D. vulgaris, promoting the transforma- tion of overgrazed communities into ones dominated by erect macroalgae. However, in the marine reserve on Ustica Island (SW Italy) the opposite trend has occurred in the upper infralittoral commu- nity, and urchin barrens formed after the cessation of fishing activities. We hypothesized that (1) the natural scarcity of the 2 seabream species leads to a low predation rate on sea urchins at Ustica, and (2) predation rate varies with depth, due to differences in the predator assemblages. Tethering experiments were conducted to test these hypotheses by comparing the predation rates at different depths at Ustica and 2 other locations in Sicily characterized by erect macroalgae communities and low urchin densities. Differences in the assemblages of predators, relative vulnerability of the 2 spe- cies of sea urchin present (Paracentrotus lividus and Arbacia lixula), and sea urchin density were also assessed. Despite large variation in predator abundance among locations, there was no difference in predation rate on tethered urchins. Predation by fish was found to be higher in locations where seabreams were abundant, while predation by the starfish Marthasterias glacialis, usually consid- ered a minor sea urchin predator, was found to be the major source of mortality at Ustica. The paucity of predatory fish at Ustica was consistent with the high abundances of urchins and persistence of barrens habitat. The high predation rate on urchins by M. glacialis at Ustica suggests that this species might play an important role in controlling urchin populations in the future.

Grazing effects of the periwinkle Echinolittorina peruviana at a central Peruvian high rocky intertidal

Echinolittorina peruviana is the most common gastropod in the high intertidal zone of Peru, representing more than 80% of the individuals present at that zone. Experimental removal of snails was used to evaluate their effects on (a) abundance of epilithic biofilm, (b) barnacle recruitment, and (c) abundance of macroalgae under “normal” conditions of the El Nino Southern Oscillation (ENSO). Experiments were carried out from October 2005 to April 2007 at two intertidal levels of a semi-protected rocky shore of central Peru. Results demonstrated that E. peruviana is able to control biofilm abundance and barnacle recruitment at both heights investigated, with marked effects in the lower zone. Erect macroalgae (Ulva spp. and Gelidium spp.) were less affected by grazing; but negative effects were observed on macroalgal crusts. Season and physical stress seem to play a more important role in the abundance of macroalgae in the high intertidal. Our results are similar to those reported elsewhere for high shore littorinids and represent baseline data to understand how the role of intertidal consumers will vary under the cold (La Nina) and warm (El Nino) phases of ENSO on these shores.

Patterns of algal recovery and small-scale effects of canopy removal as a result of human trampling on a Mediterranean rocky shallow community

The ecological importance of marine algae is widely known but in shallow coastal areas the composition and structure of algal communities may be affected by different human activities. Recovery from different trampling disturbances of two competing morphological groups (i.e. macroalgae and algal turfs) and effects of macroalgal canopy removal on the dominant associated fauna were examined using controlled trampling experiments. Six months after trampling disturbance was removed, the two morphological groups closely resembled control (untrampled) conditions, both in terms of cover and canopy (%). In particular, macroalgal recovery seemed to be very rapid: the higher the impact on the system the more rapid the recovery rate. In the short-term, the removal of macroalgal fronds (i.e. canopy) caused evident changes in invertebrate and crypto-benthic fish densities although these indirect effects were species-specific. Erect macroalgae are very sensitive to disturbance and even relatively low intensities of human use may be non-sustainable for this shallow assemblage. The present findings suggest some interesting options for the management of Mediterranean rocky shallow areas. This is crucial for coastal areas that are intended to be maintained in natural condition for conservation purposes.

The role of fishes in the organization of a Mediterranean sublittoral community.: I: Algal communities

Observational studies and experimental manipulations of grazing intensity were used to examine the role of herbivorous fishes in determining abundances and seasonal variations of algal species in a sublittoral algal community in the NW Mediterranean Sea. Three sites with different grazing pressures were compared, two with high herbivorous fish densities located in the Medes Islands Protected Area (NE Spain), where fishing is prohibited, and one with low fish abundance on the unprotected coast opposite. The algal community at the three sites exhibited marked seasonal variations in biomass, attaining peak biomass levels in late spring. Within the Protected Area, a habitat heavily grazed by fishes was characterized by the seasonal proliferation of the unpalatable red alga Asparagopsis armata Harvey, and by the dominance of a turf of epiphytic and small erect thalli of articulated calcareous algae most of the year. In contrast, nearby unprotected habitats were characterized by the seasonal occurrence of dense stands of several fleshy macroalgal species and by the dominance of well-developed articulated calcareous algae most of the year. Experimental reduction of fish grazing rapidly and dramatically altered patterns of algal abundance and dynamics. After 5 weeks of reduced herbivory, several fleshy erect algal species and epiphytic algae abundance increased significantly in herbivore exclusion areas relative to unmanipulated controls. Caging led to decreased abundance of A. armata but had no effect on the abundance of encrusting non-calcareous algae or articulated calcareous algae. These results suggest that fish grazing influences algal species abundance and dynamics at one investigated site within the Medes Islands Protected Area by reducing the populations of fleshy erect macroalgae with superior overgrowth ability, and thereby facilitating the seasonal dominance of a chemically-defended algal population. However, effects of fish grazing were not so apparent at a second site with lower abundance of fish. This preliminary study provides a basis for reevaluating the conclusion that in the Mediterranean, like in other temperate seas, fish grazing has negligible effects on benthic communities.